anti-aliasing

A technique used on a grey-scale or colour
bitmap display to make diagonal edges appear smoother by
setting pixels near the edge to intermediate colours
according to where the edge crosses them.

The most common example is black characters on a white
background. Without anti-aliasing, diagonal edges appear
jagged, like staircases, which may be noticeable on a low
resolution display. If the display can show intermediate
greys then anti-aliasing can be applied. A pixel will be
black if it is completely within the black area, or white if
it is completely outside the black area, or an intermediate
shade of grey according to the proportions of it which overlap
the black and white areas. The technique works similarly with
other foreground and background colours.

"Aliasing" refers to the fact that many points (which would
differ in the real image) are mapped or "aliased" to the same
pixel (with a single value) in the digital representation.